Recent advancements in Large Scale Integration (LSI) techniques have made possible the integration of a host of circuit functions on a single integrated chip. Indeed, according to modern fabrication techniques, it is possible to fabricate as many as 500,000 transistors on a single one centimeter square silicon chip. The high density packing of many circuits on a chip is met with the attendant problems of cross-talk and electric isolation between the circuits.
Dielectric isolation techniques have become well known in the semiconductor fabrication art for providing electrical isolation between various circuits on a chip. Specifically, the provision of oxide-filled trenches in a semiconductor substrate provides "tanks" of appropriately doped materials isolated from other similar tanks. These trenching techniques have been developed to the extent that circuits may be separated by tank walls of only two to three microns wide. This small dielectric spacing provides both an increase in the circuit densities, as well as a reduction in electrical interference between circuits.
Various prior trenching techniques are disclosed U.S. Pat. Nos. 4,318,751 to Horng; 4,356,211 to Riseman; 4,389,281 to Anantha et al; 4,473,598 to Ephrath et al; and 4,506,435 to Pliskin et al. In the noted patents, trenches are formed in a P- substrate having N+ and N- processed layers thereon. With the processes and structures disclosed in the noted patents, it is seen that the basic substrate block of P-, N+ and N- layers are formed before the trenching is done. Therefore, the bottom P- substrate becomes inaccessible from the top side for providing an ohmic contact thereto. The provision of an ohmic contact on the bottom of the substrate is possible; however, such an approach leads to a complicated metallic lead frame and mounting procedure.
There is therefore a need for an improved technique for making ohmic connections to the substrate material in trenched semiconductor structures.
There is also concomitant need for a semiconductor structure employing dielectric isolation tanks wherein a deep trench is initially formed into an unprocessed P- substrate, thereby leaving such substrate material near the top surface of the structure.